Field of the Invention
Embodiments of the present invention relate generally to high-capacity energy storage devices and methods and apparati for fabricating high-capacity energy storage devices. More specifically, methods and apparati for material spray deposition of high solid percentage slurries for forming battery active materials are disclosed.
Description of the Related Art
High-capacity energy storage devices, such as lithium-ion (Li-ion) batteries, are used in a growing number of applications, including portable electronics, medical devices, transportation, grid-connected large energy storage, renewable energy storage, and uninterruptible power supplies (UPS).
Li-ion batteries typically include an anode electrode, a cathode electrode and a separator positioned between the anode electrode and the cathode electrode. Lithium is stored in the active materials in the electrodes. The active electrode material in the positive electrode of a Li-ion battery is typically selected from lithium transition metal oxides, such as LiMn2O4, LiCoO2, LiFePO4, LiNiO2, or combinations of Ni, Li, Mn, and Co oxides and includes electroconductive particles, such as carbon or graphite, and binder material. Graphite and MCMB (meso carbon micro beads) are usually used as the active electrode material of the negative electrode, such electrode having a mean diameter of approximately 10 μm. Lithium-intercalation MCMB or graphite powder is dispersed in a polymeric binder matrix. The typical polymers for the binder matrix include PVDF (Polyvinylidene fluoride), SBR (Styrene-Butadiene Rubber), CMC (Carboxymethyl cellulose). The polymeric binder serves to bind together the active material powders to preclude crack formation and prevent disintegration of the active material powder on the surface of the current collector, as well as for good adhesion to the substrate. The quantity of polymeric binder may be in the range of 2% to 30% by weight. The separator of Li-ion batteries is typically made from microporous polyolefin polymer, such as polyethylene foam, and is applied in a separate manufacturing step.
For most energy storage applications, the charge time and capacity of energy storage devices are important parameters. In addition, the size, weight, and/or expense of such energy storage devices can be significant limitations.
One method for manufacturing anode electrodes and cathode electrodes for energy storage devices is principally based on slit coating of viscous solvent-based powder slurry mixtures of cathodically or anodically active material onto a conductive current collector followed by prolonged heating to form a dried cast sheet. A slow drying process is needed in order to prevent cracking in thick coatings and, as a result, the length of the dryers must be very long. The thickness of the electrode after drying which evaporates the solvents is finally determined by compression or calendering which adjusts the density and porosity of the final layer. Slit coating of viscous slurries is a highly developed manufacturing technology which is very dependent on the formulation, formation, and homogenation of the slurry. The formed active layer is extremely sensitive to the rate and thermal details of the drying process.
Among other problems and limitations of this technology is the slow and costly drying component which requires both a large footprint (e.g., up to 70 to 90 meters long) at coating speeds of 5-40 meters/min, and an elaborate collection and recycling system for the evaporated volatile components. Many of these are volatile organic compounds which additionally require an elaborate abatement system. Further, the resulting electrical conductivity of these types of electrodes also limits the thickness of the electrode and thus the energy density of the battery cells.
Accordingly, there is a need in the art for high volume, cost effective manufacturing processes and apparatus for manufacturing high-capacity energy storage devices.